Procedure and means for measuring with aid of an x-ray tube the distribution of fillers or equivalent in a web

ABSTRACT

A procedure and means for non-destructive measuring of the distribution in the thickness direction of the filler and/or coating materials in paper or cardboard. Radiation emitted by an x-ray tube is used to excite in the material component to be examined, its characteristic x-ray radiation, the intensity of this radiation being observed. Measurements are made on both sides of the specimen. The contents of other filler components are also determined by x-ray absorption measurements, and the base weight of the paper, e.g. by beta radiation absorption. Measurements of the characteristic radiation, elicited with constant energy x-ray radiation as well as absorption measurements of radiation obtained directly from the x-ray tube and of radiation produced in transformation targets are made in order to eliminate by calculation the disturbing interaction of the filler components. Alternatively, distribution measurements are made with the aid of characteristic radiation elicited with radiation from the x-ray tube varying in a known manner during the measuring cycle and by measuring its absorption followed by equivalent calculations.

BACKGROUND OF THE INVENTION

The present invention concerns a method for measuring the distributionin the thickness direction of filler and/or coating materials of paper,cardboard, or the equivalent, and the contents of said materials withoutdestroying the sample. In the method or procedure of the invention theradiation emitted by an x-ray tube is used to excite in the materialcomponent under examination of the object under measurement itscharacteristic x-ray radiation and the intensity of this radiation isobserved. In this procedure, measurements are carried out on both sidesof the specimen under examination. In addition, the contents of otherfiller components are determined by x-ray radiation absorptionmeasurements, in order to eliminate the effects of such componentsinterfering with the distribution measurement, and the base weight ofthe paper is determined by beta radiation absorption measurement, or byanother equivalent procedure.

Furthermore, the present invention concerns apparatus applying themethod and novel uses of the procedure and the apparatus.

When paper and paper machines are discussed in the following, referenceis generally made both to paper and cardboard, and respectively both topaper and cardboard machines.

Fillers, which as a rule are mineral substances, are incorporated in thepaper primarily for their effect of improving the printing technologicalproperties. Fillers are most commonly used for printing papers. Thefiller addition improves the opacity, lightness, printer's inkabsorption and surface smoothness of the paper. The fillers influence ina particularly advantageous manner the quality of paper to be glazed.

It is known in the art to add filler material in two ways, either bymass filling or by coating. In the mass filling method or procedure, thefiller material is added in the form of suspension to the pulp sludgebefore the arrival of the sludge on the paper machine, whereby thefiller material is admixed with the entire fiber material in thefinished paper. In the coating procedure, a suitable glue substance isadmixed with the filler material in the aqueous phase, such as starch orcasein, whereafter the surface of the paper is brushed with this mixturein a continuous process.

The filler materials in paper tend to be non-uniformly distributed inthe thickness direction of the paper, causing one-sidedness of thepaper. The one-sidedness of paper manufactured on Fourdrinier machinesis due to the fact that the fillers are washed out together with thewater that is drained, from the lower part of the pulp web into thedrainage water, whereby they become enriched in the upper part of theweb. As is known in the art, efforts have been made to reduce theproblems of one-sidedness, not only by additives improving theretention, but also by gentle dewatering at the initial draining phase,which requires a longer dewatering time and therefore implieslengthening the wire section or reducing the speed of the paper machine.

In machines with a planar wire, the difficulties with the fines andfiller distribution manifest themselves when papers for offset printingare manufactured. A high filler and fines content on the top surface ofthe paper caused dusting, which is a serious detriment in the offsetprocess. In contrast, papers manufactured on a twin wire machine areconsidered well appropriate for offset printing. This is due to thesymmetrical shape of the fines distribution and to equal leaching ofboth surfaces of the web due to two-sided dewatering. It is in factgenerally held that due to move uniform fines distribution, the printingby offset on paper manufactured on a twin wire machine is moresuccessful than that on paper manufactured on a Fourdrinier machine.Offset printability has indeed increased in significance because offsetprinting is increasingly replacing the letterpress printing procedure.

On the other hand, the filler content of the surfaces of the paper webcannot always be brought to desired level with a twin wire former. Evenwhen planar wires are used, only the top side of the web (the sidefacing away from the wire) is satisfactory as to its filler content. Thelow filler content of the web surfaces is particularly problematic inso-called SC gravure papers. Attempts may be made to increase the fillercontent of the paper surfaces by increasing the filler content of thepulp in the headbox, but even with this expedient, a satisfactorycondition is not achieved, because of the aformentioned poor retentioncharacteristic of the filler and of its enriching in the inner parts ofthe paper. In addition, when the filler content in the headbox has to beincreased, the consistency in the headbox is likely to become excessiveso that it impairs the formation of the paper.

Modern high-speed printing presses impose particularly high requirementson the printing paper. These requirements are based on trouble-freeoperation of fast printing presses and on the appearance of theprinting. The imprint is considerably influenced by the symmetry betweenthe sides of the paper and the quality of the surfaces of the paper,which is naturally also influenced by the distribution of the fillers.Heretofore, no methods or procedures and apparatus have been in use withwhich the filler distribution could have been measured even on lineeither on the paper machine, on the printing press or on the papercoating means.

It is known in the art, as described in Finnish Pat. No. 40587,inventors Juhani Kuusi and Antti Lehtinen and applicant Valmet Oy, toexcite the characteristic x-ray radiation of the filler material byradiations, such as alpha, beta, gamma or x-ray radiation, penetratingto various depths in the paper, and in this way to gain information onthe vertical distribution of the filler. The procedure has beendescribed in greater detail in a paper by J. Kuusi, entitled"Determination of Content and Distribution of Filler and CoatingMaterials in Paper Using Radioisotope X-Ray Spectrometry," Paper andTimber No. 4A, 1970. As was observed in the paper, variations inrelation to each other of the filler contents cause certain effects ofwhich the quantitative elimination by the procedures described in thepaper is impossible. This has impeded the introduction to practice ofsuch procedures.

The state of art regarding filler measurements is illustrated in generalby a publication of April, 1982 by Buchnes A., McNeiles L. A. and HewittJ. S., entitled "The Application of X-Ray Absorption and FluoresceneAnalysis to the Measurement of Paper Additives," Int. J. Appl. Radiat.Isot. Vol. 33, pp. 285 to 292 (1982), where a fluorescene and absorptiontechnique is used for determining the total contents of differentfillers, based on the assumption that the fillers are uniformlydistributed in the thickness direction of the paper. In practice, thisis hardly ever the case. It is thus understood that there are noendeavours whatsoever made in this publication to determine theimportant thickness-direction distribution, nor has it even been takeninto account as a potential source of error in determination of thetotal filler content. It should be noted, however, that in the instancesdescribed in the paper, the influence of the source of error is minimal.

Procedures capable of determining the filler distribution and the totalfiller content directly in the paper machine are not in use at all.

SUMMARY OF THE INVENTION

The principal object of the invention is to provide a new method orprocedure and apparatus suited, in addition to laboratory measurements,for the measurement of filler distribution, which method and apparatusmake possible the control and adjustment of the manufacturing process ina paper machine on the basis of filler distribution measurements.

An object of the invention is to provide a method and apparatus fordetermining the thickness-direction filler distribution in the paper andthe total filler contents either in the laboratory or directly in apaper machine, on line.

Another object of the invention is to provide a procedure and apparatusfor determining the thickness-direction filler distribution in the paperand the total filler contents when the contents of different fillercomponents, such as, for example, CaCO₃, TiO₂, kaolin, talc orequivalent, are variable.

Still another object of the invention is to provide an opportunity notonly for immediate product quality control directly in the machine, online, but also an entirely new possibility of controlling the papermanufacturing process, the significance of which is emphasized whenefforts are made to manufacture printing paper meeting over greaterquality requirements at lowest material costs. Yet another object of theinvention is to measure and control the distribution, which provides anopportunity to develop the paper machine construction and the totalcontrol systems of paper machines.

Another object of the invention is to provide a method which is suitablefor quality control of the paper fed into fast modern printing presses,and possibly for the control and/or adjustment of the operation of theprinting presses.

To achieve the aims presented in the foregoing and those which willhereinafter become apparent, in a first embodiment of the method of theinvention, the distributions or fillers and equivalent are determined bycombined processing of the two following sets of measurements.

1. Absorption meansurements for determining the contents of differentfiller components with radiation obtained directly from the source orproduced with its aid in appropriate transformation targets; as manymeasurements as there are filler components to be considered separateones.

2. Measurements of the characteristic radiation of the materialcomponents excited in the paper by different sources of radiation.

In a second embodiment of the method of the invention, the distributionmeasurements of fillers and equivalent are performed with the aid ofmeasurements of the characteristic radiation of the material componentsexcited in the specimen by radiation obtained from an x-ray tube andvarying in a known manner during the measuring cycle, and with the aidof absorption measurement of the same radiation, such absorptionmeasurement being for use in elimination by calculation of thedisturbing effect of the variations of the filler components' contentsin relation to each other.

A first embodiment of the apparatus of the invention in turn mainlycomprises a measuring head having an x-ray tube emitting constant energyradiation and a transfer mechanism therefor, and radiationtransformation plates and transfer mechanisms therefor, and radiationdetectors and pre-amplifiers. The measuring head is connected to ameasuring device having power sources, an amplifier and a counter,processor and display unit.

A second embodiment of the apparatus of the invention comprises ameasuring head with an x-ray tube emitting radiation varying in energyduring the measuring cycle in a known manner, and radiation detectorsand pre-amplifiers. The measuring head is connected to a measuringapparatus comprising power sources, amplifiers and a multi-channelcounter, processor and display unit using a time axis.

The method or procedure and apparatus hereinbefore described are used,as taught by the invention, for example, in a paper machine, in on-linemeasurement for measuring the filler distribution in the thicknessdirection and the total filler content of paper. In addition, theobtained measurement results may be used as feedback signals in thecontrol system of the paper machine, in the control of the fillerdistribution, and/or of the total filler content of various fillermaterials. An advantageous application of the invention is inmeasurement, and possibly in the control, of the coating materialcontent and/or coating material distribution in paper or cardboard thatis either being coated in an on-line process or has been tested in aseparate coating apparatus, in particular of its one-sidedness. of thepaper being fed into a printing press and/or governing, and possiblycontrolling, the operation of the printing press.

As has in part become apparent from the foregoing, the inventive idea isthat the intensity of the characteristic x-ray radiation of the fillercomponent excited with different radiation sources, and possibly withdifferent angles of incidence of the exciting radiation, is measured onboth sides of the paper. This intensity provides information about theshape of the distribution. In addition, in this x-ray fluorescencemeasurement it is possible to determine the intensity of the excitingradiation scattered back from the paper and which correlates, forexample, with the base weight of the paper. This would serve as anauxiliary quantity in the processing of results. What is significantfrom the point of view practical applications is that the contents ofvarious filler components are measured by x-ray absorption measurements.These measurements make use of the primary radiation, possibly varyingas to its energy with reference to time during the measuring cycle,emitted by an x-ray tube and radiation with desired absorptionproperties which has been derived therefrom, or from an x-ray tubeplaced on the other side of the paper, with the aid of appropriatetransformation targets. The auxiliary quantity is the absorptionmeasurement signal of beta radiation used as routine in measurements onpaper for determinations of base weight in g/m² (fibers plus filler).Based on the results of the absorption measurements, it is possible bycalculation to eliminate the effects of the variation of the differentfiller components' contents on the fluorescence measurements, and inthis manner to determine the filler distribution and the contents ofdifferent filler components.

In the laboratory, the invention affords an opportunity for a rapidquality control of the paper, and thereby for the control of themanufacturing parameters with a given lead time. Particularly the fillerdistribution close to the surface layers of the paper has considerablesignificance concerning the printability of the paper. Furthermore, adistribution of proper shape provides an opportunity to use filler inabundance, thereby lowering the total material costs. The procedures ormethods presently used in laboratories, such as dividing the paper intodifferent layers by a tearing tape, incineration of layers and ashdetermination, are slower by one order of magnitude and more inaccuratethan the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description, taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a graphical presentation of a typical filler distribution inpaper manufactured on a Fourdrinier machine;

FIG. 2 is a graphical presentation of mass absorption coefficients ofsome mineral filler and coating materials of paper, of water and ofcellulose for low energy x-ray radiation;

FIG. 3 is a schematic diagram illustrating the main principle of thefluorescence measurement of the invention;

FIGS. 4A and 4B are schematic diagrams presenting the principle of thefluorescence measurement of the invention with two different angles ofincidence of the exciting radiation and angles of departure of theexcited radiation;

FIG. 5A is a graphical presentation of the variation of the averageenergy of the radiation emitted by an x-ray tube used in accordance withthe first embodiment of the invention during the measuring cycle, as afunction of time;

FIG. 5B is a graphical presentation of the count frequency recorded bythe counter connected to the detector, in absorption measurements duringone measurement cycle, as a function of time;

FIG. 5C is a graphical presentation of the count frequency of thefluorescence signals (Ca K line) during one measuring cycle as afunction of time. In FIG. 5C, the solid line graph represents the resultof measurement obtained from the top side of the paper shown in FIG. 1,and the interrupted line graph represents the corresponding result ofmesurement obtained on the wire side;

FIG. 6A is a graphical presentation of the distribution of fillercomponents prior to coating the paper;

FIG. 6B is a graphical presentation of the same paper after the coatingprocess;

FIG. 7A is a schematic diagram of an embodiment of the apparatus of theinvention and its measuring head, wherein an x-ray tube emittingconstant energy radiation is applied;

FIG. 7B is a schematic diagram of a second embodiment of the apparatusof the invention in an arrangement by which absorption measurements arecarried out with x-ray radiation of different energies;

FIG. 7C is a schematic diagram of a third embodiment of the measuringapparatus of the invention and its measuring head, wherein an x-ray tubeemitting radiation of varying energy during the measuring cycle isapplied;

FIG. 7D is a schematic diagram of a fourth embodiment of the apparatusof the invention, utilizing the beta absorption measurement by which theauxiliary quantity required in the invention is produced;

FIG. 8 is a schematic diagram of a measuring head disposed on atransverse measuring beam for reciprocating movement over the paper web.

DESCRIPTION OF PREFERRED EMBODIMENTS

A typical filler distribution of paper in its thickness direction x isshown in FIG. 1. The filler is least in quantity on the wire side. Inthis instance, the maximum is reached slightly above the center point ofthe paper, marked 0.5 on the horizontal axis. The filler contentdecreases towards the top surface (x=1).

The attenuation or extinction of x-ray, gamma and beta radiation inmatter can generally be expressed by the exponential formula:

    I=I.sub.o e.sup.μm,

where I (l/s) is the intensity of the radiation that has gone through amass course m (g/cm²), I_(o) (l/s) is the original intensity of theradiation and μ (cm² /g) is the absorption coefficient representing theextinction or attenuation capacity of the material.

The absorption coefficients for low energy (1 to 10 keV) x-ray and gammaradiation of materials which are important in view of fillermeasurements are set forth in FIG. 2, plotted over energy. Both are thesame type of electromagnetic radiation. In FIG. 2 the abscissarepresents the energy (in keV) and the ordinate represents theabsorption coefficient μ (in cm² /g). With the exception of a fewdiscontinuous irregularities, the absorption coefficient, and thereforealso the attenuation in the material, decreases with decreasing energy.However, some of the discontinuous jumps seen in the curves of FIG. 2are of central importance in the embodiments of the invention. If thegraph of the absorption coefficient of calcium carbonate (CaCO₃) isscrutinized, we find that it descends smoothly throughout the range from1 to 4 keV, until, at 4.04 keV energy, its value discontinuouslyincreases to be tenfold and thereafter once more decreases smoothly withincreasing energy of the radiation. The physical cause underlying thisjump is that: in the range under consideration, x-ray and gammaradiation are attenuated in the material in the manner that the energyof the radiation quanta transfers totally to electrons in the atoms.Such electrons, by virtue of the energy imparted to them, are flung outfrom the atom, leaving behind a vacancy in the electron shroud. Theenergy of the x-ray or gamma quantum has to be higher than the bindingenergy holding the respective electron to its atom. When the energy ofthe radiation is lower than the 4.04 keV corresponding to the jump inthe graph for CaCO₃, the radiation is not able to detach the electronsof its inner shell (the K shell), which are the electrons most stronglybound to the atom, from the calcium atom. When the energy of theincident radiation surpasses this limit, its quanta can become absorbedin the substance by detaching electrons from the inner shell, and thisexactly gives rise to the discontinuous increment of the absorptioncoefficient. The higher the atomic number of a substance, in practice,usually the heavier it is, the higher is the energy at which is foundthis K absorption limit, that is, the absorption limit corresponding tothe K shell.

Thus, it is shown in FIG. 2 that the K absorption limit, due totitanium, of titanium dioxide (TiO₂) is located at an energy of 4.96keV. In talc and kaolin, the element with the highest atomic number issilicon (Si), and therefore the absorption coefficient decreasessteadily after the absorption limit of silicon at 1.8 keV withincreasing energy of the radiation.

It is thus understood that when radiation having an energy higher thanthe K absorption limit of calcium is directed on a substance, forexample, calcium, vacancies will form on the inner electron shells ofthe atoms. When these are filled by electrons falling from outer shells,the substance emits its characteristic K x-ray radiation, the energy ofwhich because of recoil losses is slightly lower than the energy of theK-absorption limit. The strongest line of the calcium K has energy 3.69keV, which has also been indicated on the energy axis in FIG. 1.

The characteristic x-ray radiation of each element produced throughabsorption is utilized in a manner known in the art in x-rayfluorescence analyses for determining the chemical composition of thespecimens being analyzed. In the invention, the absorption is utilizedtowards determining the filler content of the paper's different layersand thus towards determining the filler distribution. In order that thedetermination of the distribution could be made sufficiently free oferror from the viewpoint of the practical applications, the totalcontents of the different filler components in the paper must be known.This is determined, in the invention, by absorption measurements.

If, in the absorption measurements, the attenuation or extinction causedby paper containing filler is measured with two radiation energies whichare as close as possible to the absorption limit of a given component inthe manner that one energy is above and the other below the limit, thedifference in the attenuation or extinction caused by the paper willfurnish information about the content of such filler component. If thepaper contains kaolin, talc, calcium carbonate and titanium oxide asfillers, the difference in the attenuation or extinction of the K lineof manganese (5.9 keV) and of the K line of titanium (4.51 keV) willfurnish information primarily about the titanium dioxide content (FIG.2), the difference in the attenuation or extinction of the differencesof the 4.51 keV (Ti K) and 3.69 keV (Ca K) radiations will furnishinformation primarily about the CaCO₃ content, and the absoluteattenuation or extinction of the 3.69 keV radiation, primarily about thecombined content of talc and kaolin, these latter components havingabsorption components which at the last-mentioned point are clearlyhigher than the absorption coefficients of any other components of thepaper, as shown in FIG. 2.

In order to determine the contents of various filler components ofpaper, it is necessary to know the base weight of the whole paper, thatis, its mass per unit area (in g/m²). This is found by measuring theattenuation or extinction in the paper of beta radiation, for example,from an ⁸⁵ Kr source. This is because the different components of papercause equal attenuation or extinction of beta radiation, that is, ofelectrons thrown out by nuclei. The use of beta radiation fordetermining the base weight of paper is known in the art of papertechnology and is completely routine in nature.

The fluorescence measurement used for the actual determination of thefiller distribution is described more specifically with reference toFIG. 3, in connection with which it is assumed that the base weight ofthe paper specimen 10 is 100 g/m² and that it contains, as uniformlydistributed filler, 25% calcium carbonate. As shown in FIG. 3, theexciting radiation I_(e) from the source 20, which is an x-ray tube,impinges on the paper specimen at an angle of incidence α and excites inthe specimen 10 the characteristic radiation of calcium, of 3.69 keV.The detector 30 measuring the radiation I_(f) observes the radiationdeparting at an angle β from the surface 11 of the specimen 10. Sincethe exciting radiation I₃ is attenuated as it proceeds in the paperspecimen 10, it excites calcium radiation more efficiently in theadjacent top surface 11 which, which is to the source 20, than in thelower, or back, surface 12. Since the excited characteristic radiationof calcium is also attenuated in the specimen 10 to a given extent, theradiation excited adjacent the top surface 11 has easier access to thedetector 30. Both of the just-mentioned circumstances act in thedirection that the greater part of the radiation detected by thedetector 20, in the case of homogeneous filler distribution, comes fromthe top layers of the specimen 10, and therefore the topmost layers ofthe paper will be emphasized in the information thus obtained. Thesmaller the angles of incidence and departure α and β of the radiation,the greater are the differences in path length between the top surface11 and the lower surface 12, and the greater is said stress placed onthe top surface in the information gained by the detector 30. In thismanner, it is possible, by varying the angles of incidence and departureα and β, to change the relative weight factors of different layers inthe information that is measured. This is demonstrated by FIGS. 4A and4B and by the following Table 1.

                  TABLE 1                                                         ______________________________________                                        Angle of incidence       80°                                                                              30°                                 of the radiation (α)                                                    Angle of departure       80°                                                                              30°                                 of the radiation (β)                                                     Relative intensity                                                                            Depth    Intensity Intensity                                  of information from differ-                                                                   0.05     0.93      0.86                                       ent depths in the paper                                                                       0.5      0.47      0.22                                                       0.95     0.23      0.06                                       ______________________________________                                    

Table 1 shows the relative intensity of the information received influorescence measurements at various depths in the specimen when twodifferent pairs of angles of incidence and of departure, α,β of theradiation are used. The energy of the exciting radiation is equivalenton the average to the K line of manganese (5.9 keV). The base weight ofthe paper is 10 g/m² and its CaCO₃ content is 25%, assumed in thiscalculation example to be uniformly distributed in the verticaldirection. On the depth scale, the surface is denoted by coordinate 0and the back side of the paper is denoted by the value 1, making thecoordinate of the center 0.5.

The intensity values calculated in Table 1 reveal that the informationis strongly weighted in favor of the top side, in other words,emphasizing the side at which the measurement is performed. This effectis considerably strengthened upon changing the angles of incidence anddeparture of 80° in FIG. 4B one transfers to the angles α,β of 30° inFIG. 4A. This is seen when, for example, the values of the intensitiesobtained from the center of the paper (0.5) are mutually compared (0.47and 0.22).

Another manner of varying the relative weighting of different layers ofthe specimen 10 is to change the energy of the radiation used forexcitation, that is, of the radiation emitted by the x-ray tube 20. Whenthis is done in the usual manner within a given measuring cycle, thefluorescence and absorption information required for determining thedistribution is obtained simultaneously. This case will be examined ingreater detail hereinafter.

If the distribution of a given filler component in the thicknessdirection of the specimen 10 is not uniform, but, for example like thatshown in FIG. 1, the intensities of the characteristic radiation ofcalcium measured on different sides of the paper are unequal and theirdifference reflects the one-sidedness of the distribution. In the caseof a paper having a distribution substantially like that of FIG. 1 andwith a base weight of 160 g/m² and a calcium carbonate content of about20%, with 5.0 keV radiation and using angles of incidence and ofdeparture α,β=80° on the average, the ratio of 470/410 of theintensities on different sides (top side/wire or lower side) was found.When the angles of incidence and departure were reduced, the ratioincreased as could be expected. An effect in the same direction wasachieved by using softer radiation of 4.5 keV.

The determination of the filler distribution on the basis of the resultsof measurement shall be considered.

The basic distribution as in FIG. 1, can be mathematically reprented bya polynomial y=ax² +bx+c, where y refers to filler content (ordinate)and x to the coordinate in the vertical direction of the paper(abscissa). The coefficients a, b and c are found by fitting to areference distribution. The intensities of the characteristic radiationof calcium are determined from both sides of a paper with referencedistribution to serve as reference values, and is the x-ray absorptionof the paper at a suitable energy, and the beta absorption, such as, forexample, a ⁸⁵ Kr source.

Then, when the equivalent quantities are measured from an unknownspecimen belonging to the same paper brand, the differences between themand of the quantities measured from the reference paper will yield thefiller distribution of the paper sample being measured, by mathematicalmethods, utilizing the known absorption coefficients of the differentcomponents. In the vicinity of the reference distribution, a measurementcarried out with merely one pair of angles α,β, or with one energy ofthe exciting radiation, provides a rather reliable estimate of thedistribution. The reliability and accuracy can be increased by varyingthe angles of incidence and departure α, β, or by using severaldifferent radiation energies. This naturally causes the mathematicalprocessing to be more complicated.

In a case which was studied, the reference polynomial representing thefiller distribution was found to be y=-42x² +52.1x+6.7, the unit y andof the coefficients a, b and c being the CaCO₃ content in %. It followsthat the CaCO₃ according to the reference distribution, is 6.7% on thewire surface 12 (x=0) of the specimen 10, and 16.8% on the top surface11 (x=1).

After the measurement results for the intensity I of the characteristicradiation of calcium, where I₁ is the wire side 12 and I₂ is the topside 11, and the result of the x-ray absorption measurement T for thepaper specimen under examination have been corrected by applying thereference graph, with the aid of the results of the beta absorptionmeasurements to correspond to the base weight of the reference paper,the changes Δa,Δb,Δc of the coefficients of the distribution polynomialfor the paper under examination can be calculated from the system ofequations calculated from the reference polynomial. ##EQU1##

In the system of equations, ΔI,ΔIa and ΔT correspond to the values ofthe paper specimen 10 under examination and to those measured from thereference paper.

In tests that have been carried out, the new coefficients obtained fromthe system of equations were found to yield distributions in agreementwith the distributions determined from the same paper specimens byactivation analysis close to the reference distribution. It is obviousthat a more accurate approximation is attained by a greater number ofmeasurements, but the accuracy afforded by the procedure described inthe foregoing is adequate in certain supervision applications.

If in the example under consideration, kaolin, for example, is added tothe filler of the paper in addition to calcium carbonate, as isfrequently done intentionally or inadvertently in reused paper, etc.,the situation is significantly altered in the sense of measuringtechnology. This is because kaolin attenuates, in fluorescencemeasurements, both the exciting I_(e) and the excited I_(f) radiation,especially the I_(f) radiation, and as a result the variations of kaolincontent affect to a certain degree the calcium carbonate measurements,even if the content and distribution of the calcium carbonate should beconstant in the specimen 10. The influence of kaolin on the results ishowever calculable and can be eliminated with the aid of the knownabsorption coefficients, provided that the kaolin content in thespecimen 10 is known. This leads to the requirement of measuringtechnology that, in connection with the measurements, the contents ofkaolin and other potential filler components have to be determined. Thisis possible by using suitably selected radiation energies in theabsorption measurements, as hereinbefore described. It may be observed,in this connection, that of the commonly used fillers, talc and kaolinare materials of which the contents must be determined by the absorptiontechnique. Fluorescence measurements do not succeed in normal conditionsbecause in these substances the characteristic x-ray radiation, even ofthe heaviest element, silicon (Si), is so weak that is is excessivelyattenuated in the specimen 10, in the air space and in the windows ofstandard detectors 30. The same methods for CaCO₃ may be applied forTiO₂, which is occasionally used, with the difference, of course, thatthe K line (4.51 keV) of titanium is excited and measured.

It is thus understood that in complicated cases the determination of thethickness-direction distribution of filler requires several x-rayfluorescence measurements on both sides of the specimen 10 and severalabsorption measurements. The intensity of the exciting radiation I_(e)scattered back from the specimen 10, which correlates with severalcharacteristics of the specimen paper may be used as a kind of controlquantity in the measurements. In practice, when one is moving quiteclose to a given reference distribution, adequate accuracy is oftenachieved with rather few measurements.

If the x-ray tube 20 is so constructed that the energy of the radiationemitted therefrom can be varied during a measuring cycle T and thechange of the measuring signal, or pulses per unit time may be recordedsimultaneously as a function of the energy of the exciting radiation,the measuring activity may be considerably simplified.

The energy of the radiation emitted by the x-ray tube 20 changesadvantageously during one cycle of measurement, for example, as shown inFIG. 5A. If the paper specimen 10 contains kaolin, talc, calciumcarbonate and titanium oxide as fillers, the pulse frequency observed bythe counter 42 will vary in the radiation absorption measurements, asshown in FIG. 5B. When, at the beginning of the cycle T, the energyreaches a specific technical limit threshold, the counter 42 starts toobserve pulses, and the count frequency increases uniformly at first,with increasing energy. When the energy reaches the K absorption limitof calcium E₀ =4.04 keV, the count frequency falls after the respectivetime t_(o), due to the discontinuously increasing absorption coefficientof calcium. Thereafter, with a further increase in the energy of theradiation, the count frequency decreases smoothly until the K absorptionlimit E₁ =4.9 keV of titanium is reached, after the respective time t₁ adecrease in the count frequency ensuing, caused by the discontinuousincrease of the absorption coefficient of titanium. By using themagnitude of the step E₁ -E₀ (FIG. 5A) and the differences of countfrequency Ia_(o) and Ia₁ (FIG. 5B), and the absorption measurementcarried out by the beta radiation, for example, ⁸⁵ Kr, the base weightof the paper specimen 10 is found. The contents of calcium carbonate andtitanium dioxide and the combined contents of talc and kaolin, are alsofound for the corrections by calculation of the fluorescencemeasurements required in the determinations of distribution. Inpractice, it is difficult, or even impossible, to alter the energy ofthe radiation emitted by the x-ray tube 20 in the ideal andmonochromatic manner shown in FIG. 5A. If, however, the pulsefrequencies Ia recorded by the counter 42 on the average correspond toenergies between which the absorption jumps fall, the results will, withappropriate calibration measurements, usually furnish the fillercontents with sufficient accuracy.

In the fluorescence measurements constituting the basis for thedeterminations of distribution, the count frequency I₃ of the counter 42changes during the measuring cycle, as shown in FIG. 5C. The fillerdistribution is assumed to be the same as in FIG. 1, and the object ofexamination is the observed intensity of the calcium K line excited inthe paper specimen. The solid-line curve C₁ of FIG. 5C corresponds tothe measurement from the top side of the paper specimen (x=1), and theinterrupted line curve C₂ similarly represents the measurement from theunderside of the paper. When, at the beginning of the measuring cycle T,the energy of the exciting radiation is lower than the K absorptionlimit of calcium, no characteristic radiation of calcium at all will, ofcourse, be produced in the paper. When this limit is just surpassed atthe time t_(o), the signal measured at the top side of the specimen(x=1) will, in accordance with FIG. 5C, rise to be clearly higher thanthe signal measured on the wire side (x=0), because soft radiation "sees" more calcium on the top side. As the energy of the radiationincreases, the signals measured at the top side and at the wire side, asshown in curves C₁ and C₂, approach each other, however. Thus, even withhigh exciting radiation energies the signal measured at the top sideremains higher than that obtained at the wire side, due to theabsorption of excited radiation occurring in the paper. At the point t₁corresponding to the K absorption limit of titanium E₁ =4.96 keV, thereis a small dip in the curves C₁ and C₂ due to the increasing matricabsorption.

The pulse frequencies I_(e) selected at a suitable point in FIG. 5Ccorrespond to the count frequencies I₁ and I₂ excited with constantenergy at different sides of the paper in the preceding examples, andthey can therefore be used in the mathematical procedures presented fordetermining the distributions. This measuring technique has theadvantage that during the measuring cycle a great number of suchintensity pairs I₁, I₂ are obtained as a function of the excitingradiation. With the aid of these intensity pairs, more detailedinformation about the course of the distribution is obtained by moreadvanced mathematical considerations. Furthermore, it is naturallynecessary to carry out corrections for eliminating the effects from thefiller components' variations in relation to each other on the basis ofthe total contents determined by absorption measurements. Ashereinbefore mentioned in connection with the absorption measurements,it is difficult, or even impossible in practice, to change the energy ofthe radiation emitted by the x-ray tube 20 in such ideal andmonochromatic manner during one measuring cycle, as has been shown inFIG. 5A. However, in practice, even with a less ideal cycle T, in whichintensity pairs are obtained corresponding to a few average energies,adequate information is obtained for determining the distribution.

As the preceding consideration reveals, an exciting energy E varyingwithin the measuring cycle T supplies considerably more informationabout the distribution and about the contents of the filler componentsthan an exciting source with constant energy. However, the variableenergy imposes rather more exacting demands on the electronics of themeasuring apparatus, in that it is necessary in this instance to recorda continuously varying count frequency during the cycle T, and simplycounting the accumulated number of pulses is inadequate.

The printing characteristics of paper can be improved by coating thepaper with the same substances that are used as fillers. In this case,the contents of mineral components in the surface layers of the paperincreases greatly, as seen in FIGS. 6A and 6B hereinbefore discussed.Since the method of the invention provides information about thedistribution of the mineral components in the paper and, in particular,about their content in the surface layers of the papers, it is alsopossible to determine the amount of coating in the coating layers andthe difference in coating between the different sides of the paper bythe method of the invention without destroying the specimen. If thepaper is already coated, the filler distribution of the uncoated bottompaper naturally cannot be elicited any longer.

The measurement and apparatus of the invention is presented in FIGS. 7A,7B 7C and 8. FIG. 7A illustrates the x-ray fluorescence measurement byan x-ray tube 20 emitting radiation of constant energy. FIG. 7B showsthe equivalent absorption measurement. It is naturally required that afluorescence measurement be performed on both sides of the paper. FIG.7C shows apparatus for measurement carried out with an x-ray tube 20emitting radiation varying in energy within the measuring cycle T, asshown in FIG. 5A. The fluorescence measurement on one side of the paperand the absorption measurement may then be accomplished simultaneously.In this instance as well, a separate fluorescence measurement hasnaturally to be carried out at the other side of the paper.

The part isolated by interrupted lines in FIGS. 7A, 7B and 7C is themeasuring head 100. The measuring head 100 comprises the x-ray tube 20having a device, if any, for changing the angle of incidence of theexciting radiation I_(e), a detector 30 with pre-amplifier 31, and, inthe case of the x-ray tube 20 emitting constant energy radiation, also atransfer mechanism 22 for the radiation transformation target 21. Inlaboratory apparatus, the measuring head 100 is, for example, anenclosed apparatus on the table, into which the paper specimen 10 to beexamined is conveyed by a suitable mechanism. In an on-line apparatusaccomplishing the measurements directly in the paper machine, as shownin FIG. 8 the paper web 200 passes through the measuring head 100mounted on a measuring beam 300. The measuring head 100 may be soconstructed that it may traverse the paper web, as shown in FIG. 8.

The detector 30 principally consists of a proportional counter or ascintillation crystal. In certain instances, in particular, inlaboratory measurements, a semiconductor counter may also be used with aview to increasing the accuracy.

When an x-ray tube emitting constant energy radiation is used (FIGS. 7Aand 7B), the measuring head 100 is connected to measuring apparatus 40comprising a voltage source 41, an amplifier and a counter, processorand display unit 42, and to the power and control unit 44 of the x-raytube. A control unit 43 connected to a processor or computer 50 controlsthe performing of measurements and the processing of results. In thelaboratory version, the processor operations may, of course, be replacedby manual operations and the results may, of course, be processedmanually, or by an external computer. When using an x-ray tube 20emitting radiation varying in its energy during the measuring cycle T,it is necessary to use a multi-channel counter applying a time axisinstead of the counter 42.

The extent of the equipment external to the measuring head 100 and ofthe software and programs for the computer 50 is greatly dependent uponthe degree of automation and the standard of accuracy desired at, and onthe extent of the measuring range, that is, the number of differentpaper brands and the variation limits, within each brand, of thequantities which are measured.

FIG. 7A illustrates the exciting of the characteristic fluorescenceradiation of a filler component (CaCO₃ or TiO₂) and its measuring at theother side of the paper specimen 10. The radiation I_(e) emitted by theradiation source 20 excites in the paper specimen 10 the characteristicx-ray radiation of a given element (Ca or Ti) of a filler, part of whichis directed to the detector 30 and counted. The detector 30differentiates between the different types of radiation by their energywith such accuracy that the contribution of each radiation component canbe determined from the measured pulse height distribution bymathematical means. If it is desired to make the measurement atdifferent angles of incidence and departure of the radiation withreference to the surface of the paper specimen 10, movable collineatorsor radiation beam detectors may be used.

Since, for determining the distribution, a fluorescence measurement hasto be made at both sides 11 and 12 of the paper, in the laboratoryversion, the paper specimen 10 must be turned over, or two measuringheads 100 carrying out measurement at different sides of the paper haveto be used. When measurements are carried out directly in the papermachine, is the only possible alternative is the two heads.

FIG. 7B presents an arrangement by which absorption measurements arecarried out with x-ray radiation of different energies. The radiationfrom the x-ray tube 20 that has passed through the paper specimen 10excites a radiation in the backing plate 21 suitable for absorptionmeasurements and which passes partly through the paper specimen 10 tothe detector 30. In this instance, the signal of the radiation excitedin the paper specimen 10 by the source is admixed with the signal beingmeasured. The signal of the radiation excited in the specimen 10 reducesthe accuracy of measurement in certain cases.

FIG. 7C represents the measurement performed with an x-ray tube 20emitting radiation of varying energy during the measuring cycle. In themeasurement apparatus of FIG. 7C, the measuring of fluorescence is atthe same side of the specimen 10 where the x-ray tube 20 is located andthe absorption measurement is at the opposite side of said specimen,using an absorption detector 33 and a pre-amplifier 34 connected to saiddetector accomplished simultaneously.

FIG. 7D presents beta absorption measurement apparatus used as routinein the papermaking industry for base weight measurements and which inthe distribution measurements supplies the auxiliary quantityindispensable in the processing of the results. These auxiliarymeasurements are accomplished using a beta source 23, a detector 30 anda pre-amplifier 31 in the manner known in the art.

FIG. 8 is a schematic diagram of a measuring head 100 mounted on atransverse measuring beam 300 in a paper machine to perform on-linemeasurement traversing reciprocatingly the width of the travelling paperweb 200.

Detailed reference distributions which are indispensable fordemonstrating and proving the practical applicability of the method ofthe invention may be determined by neutron activation analysis ofmicrotome sections made of paper. The technique is described in anarticle by Kuusi J. and Lehtinen, A. J. entitled, "Neutron ActivationAnalysis of Microtome Cuts in Examination of Paper for Its FillerDistribution", Pulp and Paper Magazine of Canada, 71, No. 3 (1970).

The method and apparatus hereinbefore described are suitable for useeither in laboratory measurements or on-line measurements in a papermachine. In the on-line measurement use, the results obtained by themeasuring apparatus may be used as feedback signals for guiding and/orcontrolling the papermaking process towards implementing desired fillerdistribution. A possible application of the invention is the use of themethod or apparatus in the measurement, and possibly even in thecontrol, of the coating agent content and/or coating distribution eitherof paper or cardboard to be coated in an on-line process, or of papertreated in separate coating means, in particular of its onesidedness.Further applications of the invention may be the quality control ofpaper fed into a printing press, and even the guiding, or control, ofthe operation of a printing press for optimizing the printing qualityand minimizing trouble encountered in the operation of the printingpress.

The invention is by no means restricted to the aforementioned detailswhich are described only as examples; they may vary within the frameworkof the invention, as defined in the following claims.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above constructions withoutdeparting from the spirit and scope of the invention, it is intendedthat all matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A method of measuring, without destroying thespecimen, the distribution in the thickness direction of the fillerand/or coating materials of paper or cardboard, and the content of saidmaterials, wherein radiation emitted by an X-ray tube is used to excitein the material component to be examined, of the object of measurement,its characteristic X-ray radiation, the intensity of said characteristicradiation being observed, measurements are made on both sides of thepaper or cardboard under examination, the contents of said materials aredetermined by X-ray absorption measurements for eliminating the effectsof these components disturbing the distribution measurement, andmeasurement is made of the base weight of the paper or cardboard underexamination by radiation absorption, said method comprising the stepsofmaking a number of X-ray absorption measurements, the number ofmeasurements being at least equal to the number of different fillercomponents, for determining the contents of the different fillercomponents and the coating materials by means of constant energy X-rayradiation; making a number of measurements of the characteristic X-rayfluorescent radiation of the material components excited via the X-raytube of the paper or cardboard; and determining the distributions offillers and coating materials by calculative joint processing of theresults from said measurements.
 2. A method as claimed in claim 1,wherein radiation is obtained directly from said X-ray tube.
 3. A methodas claimed in claim 1, wherein radiation is provided by use of atransformation target.
 4. A method as claimed in claim 1, said methodfurther comprising the step of determining the intensity of theradiation from said X-ray tube scattered back from the paper orcardboard, which correlates with the base weight of the paper orcardboard, so as to provide an auxiliary quantity in the processing ofresults, in addition to X-ray fluorescence measurements.
 5. A method asclaimed in claim 1, wherein the contents of various filler componentsare measured by X-ray absorption measurements utilizing the primaryradiation emitted by said X-ray tube and radiation with specificabsorption properties derived from said primary radiation via atransformation target.
 6. A method as claimed in claim 5, wherein thefiller material of said paper or cardboard under examination isprincipally kaolin, talc, calcium carbonate and/or titanium dioxide,said method utilizing the characteristic 5.9 keV K line excited by theprimary radiation source in the manganese of said transformation target,the characteristic 4.51 keV K line excited in the titanium of saidtransformation target and the calcium 3.69 keV K line excited in thecalcium of said transformation target, and using the differenceencountered between the extinction of said characteristic 5.9 keV K linein said manganese and of said characteristic 4.51 keV K line excited insaid titanium primarily in determining the titanium dioxide content,utilizing the absorption difference observed in the extinctions of saidK line of said titanium and said 3.69 keV K line of said calciumprimarily in determining the CaCO₃ content, and using the informationprovided by the attenuation of said calcium K line primarily fordetermining the combined content of talc and kaolin.
 7. A method asclaimed in claim 1, further comprising the step of measuring theattenuation in the object under measurement of beta rays emitted by an⁸⁵ Kr source to determine the base weight in g/m² of said paper.
 8. Amethod as claimed in claim 1, wherein said measurements are carried outwith at least two angles of incidence (α) of the exciting radiationemitted by said X-ray tube.
 9. A method as claimed in claim 8, whereinsaid measurements are carried out with at least two angles of departure(β) of the characteristic X-ray radiation excited in the specimen by theradiation emitted by said X-ray tube.
 10. A method as claimed in claim9, wherein the angle of incidence (α) of said radiation is equal inmagnitude to the angle of departure (β) of the excited radiationrelative to the plane of said paper or cardboard on the same side ofsaid paper or cardboard.
 11. A method of measuring, without destroyingthe specimen, the distribution in the thickness direction of the fillerand/or coating materials of paper or cardboard, and the content of saidmaterials, wherein radiation emitted by an X-ray tube is used to excitein the material component to be examined, of the object of measurement,its characteristic X-ray radiation, the intensity of said characteristicradiation being observed, measurements are made on both sides of thepaper or cardboard under examination, the contents of filler componentsare determined by X-ray absorption measurements for eliminating theeffects of these components disturbing the distribution measurement, andmeasurement is made of the base weight of the paper or cardboard underexamination by radiation absorption, said method comprising the stepsofmaking a number of X-ray absorption measurements, the number ofmeasurements being at least equal to the number of different fillercomponents, for determining the contents of the different fillercomponents and the coating materials by means of radiation varying inenergy during a measuring cycle; making a number of measurements of thecharacteristic X-ray fluorescent radiation of the material componentsexcited via the X-ray tube in the paper or cardboard; and determiningthe distributions of fillers and coating materials by calculative jointprocessing of the results from said measurements.
 12. A method asclaimed in claim 11, wherein radiation is obtained directly from saidX-ray tube.
 13. A method as claimed in claim 11, wherein radiation isprovided by use of a transformation target.
 14. A method as claimed inclaim 11, further comprising the step of recording the variation ofintensity of an observed signal during a measuring cycle of fluorescentmeasurement and absorption measurements, as a function of the variationof the energy of the radiation emitted by said X-ray tube.
 15. A methodas claimed in claim 11, wherein the filler material of said paper orcardboard under examination is principally kaolin, talc, calciumcarbonate and/or titanium dioxide, the K absorption limit of titanium is4.96 keV and the K absorption limit of calcium is 4.04 keV, said methodutilizing the difference in the intensities of the signals correspondingto energies of the exciting radiation above and below said K absorptionlimit of titanium to determine the titanium dioxide content inabsorption measurements, utilizing the difference of the signals of theexciting radiation measured on both sides of said K absorption limit ofcalcium to determine the content of calcium carbonate, and utilizing theequivalent attenuation of the signal measured at an energy lower thanthe K absorption limit of calcium primarily to determine the combinedcontent of kaolin and talc.
 16. A method as claimed in claim 11, furthercomprising the step of measuring the contents of different fillercomponents by measuring X-ray absorption utilizing the variation duringthe measuring cycle of the intensity of the signal derived in theabsorption measurements as a function of the variation in energy of theradiation emitted by said X-ray tube when said energy varies, so thatthe range of variation of the average energy covers at least the energyrange from 3 to 8 keV.
 17. Apparatus for measuring the distribution inthe thickness direction and the content of said materials of fillerand/or coating materials of paper or cardboard, without destroying thespecimen, said apparatus including means having an X-ray tube emittingradiation which excites in the material component under examination, ofthe object of measurement, its characteristic X-ray fluorescentradiation, means for observing the intensity of said characteristicradiation, means for performing measurements on both sides of the paperor cardboard under examination and for determining the contents offiller components by X-ray absorption measurements for eliminating theeffects of these compenents disturbing the distribution measurement, andmeans for measuring the base weight of the paper or cardboard underexamination by beta radiation absorption measurement, said apparatuscomprisinga measuring unit having power sources, amplifiers, a counter,a processor and a display unit; and a measuring head connected to saidmeasuring unit, said measuring head having an X-ray tube emittingconstant energy radiation, radiation transforming plates, transfer meansfor said plates, radiation detectors and pre-amplifiers connected toeach other in a manner whereby they perform X-ray absorptionmeasurements for the determination of the contents of the differentfiller components by utilizing radiation directly from said X-ray tubeor radiation excited with its aid in suitable transformation targets andmeasurements of characteristic radiation of different materialcomponents in paper or cardboard excited by said X-ray tube. 18.Apparatus as claimed in claim 17, wherein said measuring head indisposed to traverse reciprocatingly over the entire width of thespecimen or part thereof.
 19. Apparatus as claimed in claim 17, whereinsaid measuring unit further comprises a control unit which controls thecarrying out of the measurements and the processing of results. 20.Apparatus as claimed in claim 17, wherein said radiation detectors insaid measuring head comprise proportional counters.
 21. Apparatus asclaimed in claim 17, wherein said radiation detectors in said measuringhead comprise semiconductor counters.
 22. Apparatus as claimed in claim17, further comprising a computer connected to said measuring unit, saidcomputer being programmed with a measurement result-processing andoutputting program.
 23. Apparatus as claimed in claim 17, furthercomprising a control unit connected to said measuring head, said controlunit controlling the carrying out of the measurement cycle. 24.Apparatus for measuring the distribution in the thickness direction andthe content materials of filler and/or coating materials of paper orcardboard, without destroying the specimen, said apparatus includingmeans having an X-ray tube emitting radiation which excites in thematerial component under examination, of the object of measurement, itscharacteristic X-ray radiation, means for observing the intensity ofsaid characteristic radiation, means for performing measurements on bothsides of the paper or cardboard under examination and for determiningthe contents of filler components by X-ray absorption measurements foreliminating the effects of these components disturbing the distributionmeasurement, and means for measuring the base weight of the paper orcardboard under examination by radiation absorption measurement, saidapparatus comprisinga measuring unit having power sources, amplifiers, amultichannel counter, a processor and a display unit using a time axis;and a measuring head connected to said measuring unit, said measuringhead having an X-ray tube emitting radiation varying in energy during ameasuring cycle, radiation detectors and preamplifiers connected to eachother in such manner as to determine the contents of the differentfiller components via radiation directly from said X-ray tube andvarying in energy during the measuring cycle and measure absorption andthe characteristic radiation of the material component excited in thepaper or cardboard by said radiation from said tube.
 25. Apparatus asclaimed in claim 24, further comprising a control unit connected to saidmeasuring head for controlling the carrying out of the measurementcycle.
 26. Apparatus as claimed in claim 24, wherein said measuring unitfurther comprises a control unit which controls the carrying out of themeasurements and the processing of results.
 27. Apparatus as claimed inclaim 24, wherein said radiation detectors in said measuring headcomprise proportional counters.
 28. Apparatus as claimed in claim 24,wherein said radiation detectors in said measuring head comprisesemiconductor counters.
 29. Apparatus as claimed in claim 24, furthercomprising a computer connected to said measuring unit, said computerbeing programmed with a measurement result-processing and outputtingprogram.